In certain environments, such as factories, for example, multiple motors may be used in different locations throughout the area. It is generally preferable, however, to provide a central location to provide control for all of the motors. In this situation, since the motors may be spaced widely from each other and from the central location, the distance between the central location and each individual motor may be relatively long. As is described is further detail below, the high speed switching that is commonly used to provide the signals to control the motors causes substantial noise when it is transmitted over lines that are relatively long. This noise interferes with the operation of the motors and is thus undesirable.
FIG. 1 illustrates one example of a conventional approach to motor control is such an environment. Generally, DC-Link fed motor drives use a converter-inverter approach with intermediate link control to control the speed and torque of motors used for applications like fans, blowers pumps and compressors, for example. As illustrated in FIG. 1, an AC input voltage, preferably a line voltage, may be supplied to an input rectifier 12 which rectifies the AC voltage to provide a DC voltage across DC bus voltage capacitor 13. This voltage is provided to motor 14 via the output inverter 15. A bias power supply 16 is provided to power a control and pulse width modulation (PWM) component 17 which provides PWM control for the switches S1-S6 of the output inverter 15 via the gate driver 18. That is, the switches S1-S6 are controlled such that the output voltage signal provided from the inverter 15 to the motor 14 results in the desired operation of the motor. An EMI filter 11 may also be provided to filter the line voltage.
The conventional approach to motor control described above is effective, however, as noted above, this approach results in the introduction of substantial noise. Since the motor 14 may be separated from the central control location, the voltage signal provided to the motor 14 must travel a relatively long distance. The high speed, high voltage switching that is used to provide the voltage signal supplied to the motor 14 results in substantial noise. The noise is fed back to the motor 14 and interferes with its operation. That is, reflection and distortion of current and voltage waveforms occurs, which in turn interferes with proper control of the motor 14. FIGS. 2 and 3 are graphs that illustrate the effect of noise on current and voltage waveforms. FIG. 2 illustrates the measured current waveform at either end of the line connecting the inverter 15 and the motor 14. As can be seen in FIG. 2, the current waveform shows substantial disturbance. FIG. 3 illustrates the effect of noise on measured voltage waveform at each end of the line connecting the inverter 15 and the motor 14.
Thus, it would be desirable to provide for centralized motor control while avoiding the problems mentioned above.